Documents stored on the Home screen acts much like a series of "tabbed" documents you would typically find in the desktop version. Any document shown on the screen is open (but minimised to a thumbnail) and can be either unsaved (stored internally) or previously saved to a file on local or cloud storage. Like desktop apps, you have to save your documents to preserve your edits, and then close them if you've finished with them.
If you have imported a RAW image or an image from your Apple Photos library, a saved Untitled document will always be created. For other imported documents (e.g. .afphoto or image files), the existing filename will be used.
If you need to undo your work at any time, you can use the History panel to scroll back to a history step from which to begin editing again; history steps subsequent to this step will be replaced by your new editing steps.
By default, document history is stored in the current session only, but you have the option to save the history in the document when the document is saved.
A duplicate of the document is placed alongside the original document on the Home screen.